Transboundary sources dominated PM in Thimphu, Bhutan.

This study estimates the potential source regions contributing to PM in the capital city of Thimphu, Bhutan, during the years 2018-2020 using the ground-based data, followed by the HYSPLIT back trajectory analysis. The average PM concentration in the entire study period was 32.47 µg/m which is three times of the World Health Organization recommended limit of 10 µg/m. Less than half of the days in pre-monsoon (43.47%) and post-monsoon (46.41%), and no days in winter were within the 24-h average WHO guideline of 25 μg/m. During the COVID-19 lockdown imposed from August 11 to September 21 in Bhutan, only a marginal reduction of 4% in the PM concentrations was observed, indicating that nonlocal emissions dominate the PM concentrations in Thimphu, Bhutan. Most back trajectories in the analysis period were allocated to south or south-west sector. India was the major contributor (~ 44%), followed by Bangladesh (~ 19%), Bhutan itself (~ 19%) and China (~ 16%). This study confirms that there are significant contributions from transboundary sources to PM concentrations in Thimphu, Bhutan, and the elevated PM concentrations need to be tackled with appropriate action plans and interventions.